Hi,
When we sailed the first week of December on the Mariner for their 1st Western cruise, we had cell service most of the evening sailing out of port. Key West must have a pretty good tower. Also, believe it or not we also had signal in Jamaica. My hubby placed a call from there back to our home in Florida from our chair on our balcony stateroom. We did not check our cell phone for service in Labadee, Grand Cayman or Mexico. But, we did place a call again the last night aboard the ship (day at sea #2) as we knew we would be getting close again to the Key's towers. So, I can say for sure that three of the 7 days on board the ship we were able to call home. We have AT&T, incase you were wondering. We found using the cell to be much more convient then trying to use a pay phone at the ports. For instance, in Jamacia we only saw two phones near the dock (next to the port authority office) and one of the phones was broke. You can imagine what the line looked like for the other one. We even took a $20.00 each way taxi ride to the nearest full service resort (Sandal's Resort and Spa@ Dunn River Falls...I don't reccomend this place) and they didn't even have pay phones...anywhere their property! However, in Jamaica we did see alot of advertising signs for AT&T, which is why we gave our cell phone a shot when we got back onboard the ship. Worked well...no problems with the service. I'll let ya know about the bill though as we do not have free roaming charges.
Hope this helps...Mmketeer